The invention relates to film-transport mechanisms for motion-picture projectors, and the like. The type of mechanism in question is one in which a rotary cam controls the movements of the transport claw in the direction of forward and reverse film transport, and in which the cam is so designed that one sector effects one-frame advancement in the forward-transport mode of operation, another sector effects one-frame advancement in the reverse-transport mode of operation, but the direction of rotation of the cam is the same for both modes of operation.
With transport mechanisms of this type, it has not been possible to automatically achieve frame-line equalization upon transport-direction switchovers using only a single cam follower. German published patent application DT-OS No. 2,302,803 discloses a transport mechanism utilizing two cam followers. Frame-line equalization is achieved utilizing the play of whichever of the two cam followers is out of engagement with the cam track relative to the part of the cam track located furthest from the cam rotation axis. With increasing wear, this play increases, and the shifting of the frame lines which occurs upon a reversal of transport direction becomes overcompensated, causing frame location errors to arise. Also, the manufacturing tolerances required for the mounting of the cam follower and for its point of coupling to the transport claw are disadvantageous upon direction reversals.